This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to 0002081-8 filed in Sweden on Jun. 5, 2000; the entire content of which is hereby incorporated by reference.
The present invention generally relates to optical transmission techniques, particularly single mode integrated optics, for tele and data communication.
More specifically, the invention relates to apparatus for tunable add/drop multiplexing and to methods for tunable add/drop multiplexing.
Tunable optical add-drop multiplexers are needed in different kind of networks. Especially in ring and bus configurations the add-drop functionality is vital. Add-drop multiplexers have been implemented using various principles.
A multiplexer provided with Bragg gratings and based on an MZI structure (MZI, Mach-Zehnder interferometer) is depicted in Low Crosstalk Optical Add-Drop Multiplexer based on a Planar Silica-On-Silicon Mach-Zehnder Interferometer with UV-induced Bragg Gratings and UV-Trimming, J.-M. Jouanno et al., Tech. Dig. Bragg Gratings, Photosensitivity, and Poling in Glass Fibers and Waveguides: Applications and Fundamentals, OSA, 1997, Williamsburg, Va., pp. 259-261. In principle, an add/drop multiplexer may be achieved using such a device, which comprises two directional couplers interconnected by two interference arms, a so-called MZI structure. In practice, however, possibly two separate devices are required in order to achieve complete add/drop functionality. Alternatively, two Bragg gratings may be cascaded in each Mach-Zehnder arm. Further, coupling to so-called cladding modes may occur in the grating structures, which would result in deteriorated performance of the device, particularly for channels at carrier wavelengths shorter than the Bragg wavelength.
A wavelength selectively tunable device, called MMIMZI demultiplexer, (MMIMZI, multimode interference Mach-Zehnder interferometer) may be utilized as a reconfigurable demultiplexer, see e.g. the publication A new type of tunable demultiplexer using a multi-leg Mach-Zehnder interferometer, J.-P. Weber et al., Proc. ECIO ""97 EthE5, Stockholm, pp. 272-275, 1997. Cascading of two such devices may result in a complete tunable add/drop multiplexer. Such a multiplexer has though a very narrow range within which the crosstalk performance is good. This may be solved, but then very complex interference circuits are required to achieve nonlinear phase response in the Mach-Zehnder arms of the multiplexer. Further, interference problems may occur for transmitted channels when two MMIMZI devices are cascaded.
A wavelength selective device based on an MMIMIBg structure (MMIMIBg, Bragg grating assisted multimode interference Michelson interferometer), which offers completely individual switching, is depicted in Bragg grating assisted MMIMI coupler for wavelength selective switching, T. Augustsson, Electron. Lett., Vol. 34(25), pp. 2416-2418, 1998. Even though the theory of the technique indicates low crosstalk, e.g. fabrication process dependent variations may increase the crosstalk. Possibly, two separate devices are needed to obtain add/drop-functionality.
An MMI-based device provided with Bragg gratings is disclosed in the publication Bragg Grating-Assisted MMI coupler for Add-Drop Multiplexing, T. Augustsson, J. Lightwave Technol., Vol. 16(8), pp. 1517-1522, 1998. It is, however, difficult to realize such a device, which can handle channels separated by less than 400 GHz, and still exhibit good filter performance.
It is a main object of the present invention to provide an apparatus and method for add/drop multiplexing of a wavelength channel multiplex, which are capable of handling a large number of channels simultaneously.
It is in this respect a particular object of the invention to provide such apparatus and method for add/drop multiplexing of a wavelength channel multiplex, which are capable of handling wavelength channels that are closely spaced.
It is still a further object of the invention to provide such apparatus and method for add/drop multiplexing of a wavelength channel multiplex, which exhibit tunability between a plurality of closely spaced wavelength channels.
It is yet a further object of the invention to provide an apparatus for add/drop multiplexing of a wavelength channel multiplex, which is compact and which includes short propagation lengths for the wavelength channels.
These objects among others are, according to a first aspect of the present invention, attained by an apparatus for optical add/drop multiplexing comprising an MMI coupler at least of size 4xc3x974 and an MI arm section.
The MMI coupler includes a multiplex input terminal for input of said wavelength channel multiplex into said MMI coupler; an add channel input terminal for input of a wavelength channel to be added to said multiplex into said MMI coupler; a drop channel output terminal for output of a wavelength channel to be dropped from said multiplex from said MMI coupler; a multiplex output terminal for output of said wavelength channel multiplex from said MMI coupler; and at least four input/output terminals. The MMI coupler is further effective to split light incoming on one of its input terminals into light shares delivered onto its input/output terminals, and to combine light shares incoming on its input/output terminals into combined light delivered onto one of its output terminals in dependence on the mutual phase relation of the incoming light shares.
The MI arm section includes at least four waveguides, wherein each waveguide is optically connected to a respective one of the input/output terminals of the MMI coupler, and wherein each of all, or of all but one, of the waveguides is at least provided with, as seen from the MMI coupler, a first phase control element, a narrow band reflection grating, preferably a Bragg grating, a second phase control element, and a wide frequency range reflection means.
According to the invention each of said narrow band reflection gratings is provided with a heater means, preferably a Peltier element, for tuning the respective narrow band reflection grating to the wavelength channel to be added and/or to the wavelength channel to be dropped; the first phase control elements are effective to direct the channel to be added to the multiplex output terminal of the MMI coupler and/or to direct the channel to be dropped to the drop channel output terminal of the MMI coupler; the wide frequency range reflection means are effective to reflect all wavelength channels comprised in the input wavelength channel multiplex except the wavelength channel to be dropped; and the second phase control elements are effective to direct all wavelength channels comprised in the input wavelength channel multiplex except the wavelength channel to be dropped to the multiplex output terminal of the MMI coupler.
The apparatus, and particularly the Bragg gratings, are preferably made of a material having a strongly temperature dependent refractive index, i.e. a high |dn/dT|, such as a monomer or a polymer.
In a particularly advantageous embodiment of the invention each of the wide frequency range reflection means has a reflection coefficient below 1 for the wavelength channels comprised in the input wavelength channel multiplex. The reflection coefficient may be between 0.82 and 0.98, preferably between 0.86 and 0.94, more preferably between 0.88 and 0.92, and most preferably around 0.90.
In yet a particularly advantageous embodiment of the invention, the wide frequency range reflection means are each comprised of a loop mirror device, particularly an MMI-based device.
According to a second aspect of the present invention an apparatus for optical drop multiplexing comprising an MMI coupler at least of size 3xc3x973 and an MI arm section is provided.
The MMI coupler includes a multiplex input terminal for input of said wavelength channel multiplex into said MMI coupler; a drop channel output terminal for output of a wavelength channel to be dropped from said multiplex from said MMI coupler; a multiplex output terminal for output of the remaining wavelength channel multiplex from said MMI coupler; and at least three input/output terminals. Further, the MMI coupler is effective to split light incoming on its input terminal into light shares delivered onto its input/output terminals, and to combine light shares incoming on its input/output terminals into combined light delivered onto one of its output terminals in dependence on the mutual phase relation of the incoming light shares.
The MI arm section includes at least three waveguides, wherein each waveguide is optically connected to a respective one of said input/output terminals of the MMI coupler; and wherein each of all, or of all but one, of the waveguides is at least provided with, as seen from the MMI coupler, a first phase control element, a narrow band reflection grating, a second phase control element, and a wide frequency range reflection means.
Each of said narrow band reflection gratings is provided with a heater means for tuning the respective narrow band reflection grating to the wavelength channel to be dropped; the first phase control elements are effective to direct the channel to be dropped to the drop channel output terminal of the MMI coupler; the wide frequency range reflection means are effective to reflect all wavelength channels comprised in the input wavelength channel multiplex except the wavelength channel to be dropped; and the second phase control elements are effective to direct all wavelength channels comprised in the input wavelength channel multiplex except the wavelength channel to be dropped to the multiplex output terminal of the MMI coupler.
According to a third aspect of the present invention, an apparatus for optical add multiplexing of an add channel and a wavelength channel multiplex is provided, the apparatus comprising an MMI coupler at least of size 3xc3x973 and an MI arm section.
The MMI coupler includes a multiplex input terminal for input of said wavelength channel multiplex into said MMI coupler; an add channel input terminal for input of the wavelength channel to be added to said multiplex into said MMI coupler; a multiplex output terminal for output of the wavelength channel multiplex from said MMI coupler; and at least three input/output terminals. Further, the MMI coupler is effective to split light incoming on one of its input terminals into light shares delivered onto its input/output terminals, and to combine light shares incoming on its input/output terminals into combined light delivered onto its output terminal in dependence on the mutual phase relation of the incoming light shares.
The MI arm section includes at least three waveguides, wherein each waveguide is optically connected to a respective one of said input/output terminals of the MMI coupler; and wherein each of all, or of all but one, of the waveguides is at least provided with, as seen from the MMI coupler, a first phase control element, a narrow band reflection grating, a second phase control element, and a wide frequency range reflection means.
Each of said narrow band reflection gratings is provided with a heater means for tuning the respective narrow band reflection grating to the wavelength channel to be added; the first phase control elements are effective to direct the channel to be added to the multiplex output terminal of the MMI coupler; the wide frequency range reflection means are effective to reflect all wavelength channels comprised in the input wavelength channel multiplex; and the second phase control elements are effective to direct all wavelength channels comprised in the input wavelength channel multiplex to the multiplex output terminal of the MMI coupler.
According to fourth and fifth aspects of the present invention methods for add and drop multiplexing, respectively, are provided for using the apparatus according to the first three aspects of the invention.
An advantage of the present invention is that it provides for a very effective and flexible add/drop multiplexing.
A further advantage of the present invention is that it provides for the handling of a large number of closely spaced wavelength channels.
Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention.